There is often a need to measure the weak local magnetic field generated by a local source in the presence of ambient magnetic noise fields from remote, often strong sources. An example of such a weak magnetic field is a biomagnetic field generated from sources within living creatures. The local measurement is usually modeled as the sum of a signal term (from the local source) and a noise term (from the remote source). This is usually a good approximation since the two sources are normally independent. There are three general approaches that are employed. The first approach is to carry out the desired measurements within a magnetically shielded room or enclosure, thus minimizing or eliminating the effects of the ambient noise field. A second method uses a priori knowledge or estimates of the spatial variation of the noise field to design sensors that largely reject the undesired signal. A third method forms a "real-time" estimate of the noise field at the measurement site and eliminates the effects of the noise field by "subtracting" out the noise component "passively" (electronically subtract the estimated noise signal from the sensor signal) or "actively" (by generating a field in the vicinity of the sensor site to oppose the local value of the noise field). In either case, it is hoped that the sensor output contains none of the remote noise field.
When it is desirable to be able to perform measurements without the use of a shielded room, then elements of method two and three must be used to reject and/or cancel the effects of the noise sources. The current practice is to measure the noise field (and possibly the field gradients) at a location that is a bit farther away from the measurement site (thus minimizing the signal component contribution), but not too near to the remote noise source. Based on these measurements and an a priori model of the noise field behavior, an estimate of the noise field at the measurement site is calculated. Very often, the estimate of the noise field at the measurement site is poor due to the typically high spatial nonuniformity of the noise field. In many applications such as measurements of the magnetic fields from bioelectric sources, the ambient noise levels are 10.sup.7 -10.sup.8 times stronger than that signal. Thus, any extrapolations must be extremely precise.
What is needed therefore is a method to directly estimate the noise field component at the sensor site. Thus no extrapolation would be needed to estimate the noise field at the measurement site from a more remote noise measuring site.